Fluid-operated step motor



NOV. 19, 1968 J, H. ac ETAL 3,411,413

FLUID-OPERATED STEP MOTOR Filed Nov. 16, 1966 4 Sheets-Sheet 1 60 50 &

llllllll Ill INVENTORS 5|" JOHN H.MocNE|LL a RALPH P. MEANS tJ BY M1 4 MATTORNEYS Nov. 19, 1968 J, H. M NElLL. ETAL 3,411,413

FLUID-OPERATED STEP MOTOR 4 Sheets-Sheet 2 Filed Nov. 16, 1966 MES oL LAm m I 6 Wm l l W MOP 2 W Hm N m WM J I ATTORNEY Nov. 19, 1968 Filed Nov;16, 1966 J. H. M NEILL ETAL 3,411,413

FLUID-OPERATED STEP MOTOR 4 Sheets-Sheet 3 EGA- I B3 B9 4 m 3 52 c3 BlA2 Al I27 123 ISI n9 n7 M PRESSURE |53 SOURCE INVENTORS JOHN H.MGCNE|LL8 RALPH P. MEANS BY W, 1 M

ATTORNEYS Nov. 19, 1968 J, MacNElLL ETAL 3,411,413

FLUID-OPERATED STEP MOTOR 4 Sheets-Sheet 4 Filed Nov. 16, 1966 INVENTORSJOHN H. MclcNElLL 8 P. MEANS RALPH ATTORNEYS United States Patent3,411,413 FLUID-OPERATED STEP MOTOR John H. MacNeill, Indialantic, andRalph P. Means, Melbourne, Fla., assignors to Soroban Engineering, Inc.,Melbourne, Fla., a corporation of Florida Filed Nov. 16, 1966, Ser. No.594,942 13 Claims. (Cl. 91357) ABSTRACT OF THE DISCLOSURE Afluid-operated step motor is disclosed comprising a movable memberdisposed in a housing and dividing the housing into twopressure-isolated compartments, the compartments each being suppliedwith fluid at equal pressures. A plurality of ports are defined throughthe housing and are selectively exhausted to a low pressure environment,the member being moved into alignment with a selected exhaust port bythe pressure differential developed across the member by the exhaustingof fluid from one of the compartments via the selected exhaust ports.Selection of an exhaust port is accomplished by selective actuation ofone of only three control valves, one valve controlling flow to the lowpressure environment from the first of every three sequentially disposedexhaust ports, a second valve controlling flow to the low pressureenvironment from the second of every three sequentially disposed exhaustports, and the third valve controlling flow to the low pressureenvironment from the third of every three sequentially disposed exhaustports. A masking disc rotatable with the vane permits only the portsdisposed on opposite sides of the vane to. communicate with the chamberat any given time.

This invention relates to accurate positioning devices, and particularlyto fluid-operated step motors capable of rapid bi-directional rotationalmotion.

Conventional hydraulic step motors employ the principle thatdifferential fluid pressures, acting on respective sides of a vane whichis free to move relative to the motor housing, may be utilized to movethe vane to a pre-determined position. In such a motor, a series ofexhaust ports, spaced along the housing wall, are selectively operableto effect a pressure reduction on one side of the vane or the other tocreate the required differential fluid pressures. When the vane movesinto alignment with the selected exhaust port, the port is blocked andthe fluid pressures on both sides of the vane become equalized, servingto center the vane on said port.

Devices of the type described above have been found to operate quiteaccurately, but nevertheless do exhibit certain disadvantages, one ofwhich results from the fact that an individual valve means is requiredfor each of the position-defining exhaust ports. The large number ofvalves tends to make the motor expensive and large.

The present invention, by requiring only three individual valve means tocontrol all the exhaust ports, substantially does away with theabove-mentioned disadvantages. This use of only three valves to controla step motor is achieved by manifolding the ports into three groups withthe first of every three sequentially spaced ports connected to a firstmanifold, the second of every group of three ports connected to a secondmanifold and the third of every group of three ports connected 'with athird manifold. Each manifold is provided with a valve and the valvesare operated in sequence to step the vane. To insure that the vaneadvances only to the next adjacent port a masking disc is secured to thevane so as to rotate therewith, the disc being such that when the vaneis centered over a particular exhaust port, only one port immediatelyadjacent each side of the vane Patented Nov. 19, 1968 can conduct fluidfrom the chamber. The masking disc thus permits only one exhaust port ata time to create pressure changes on respective sides of the vane eventhough one-third of all of the exhaust ports are simultaneously opened.

In addition to the single-step operation made possible by the inventionas outlined above, the step motor of this invention is also capable ofrapidly moving through a large portion of its cycle by rapid operationof the three control valves in sequence for a pre-determined number ofoperations, said number being controlled by a counting means.

It is therefore an object of the present invention to provide animproved fluid operated step motor having far fewer control valves thanthe conventional step motor.

It is another object of this invention to provide a fluid operated stepmotor which requires only three control valves for normal operation.

It is still a further object of this invention to provide an improvedfluid operated step motor employing a fluid mask secured to a rotatingvane element and requiring only three control valves to effect astepping operation.

The above and still further objects, features and advantages of thepresent invention will become apparent upon consideration of thefollowing detailed description of one specific embodiment thereof,especially when taken in conjunction with the accompanying drawing,wherein:

FIGURE 1 is a view in perspective of one embodiment of this invention;

FIGURES 2 and 3 are partial cross-sectional views taken through lines 22and 33 of FIGURE 1 and a partial schematic representation of the variouscontrols employed for that embodiment;

FIGURE 4 is a schematic representation of a second embodiment of thisinvention; and

FIGURE 5 is a schematic representation of still another embodiment ofthis invention.

Referring specifically to FIGURES 1, 2 and 3, there is illustrated oneembodiment of the step motor of the present invention wherein the motorhousing 10 comprises cylindrical wall 11 extending between two circularend walls 13 and 15 so as to form chamber 16 therebetween. Rotatablymounted in the end walls 13 and 15 and coaxial with the end walls andthe cylindrical wall 11 is shaft 17 which is connected to a load (notshown). A sector 19 extends radially between wall 19 and the shaft 17and axially between the walls 13 and 15. The sector 19 is rigidlysecured and sealed to end wall 15 and cylinder wall 11. A resilient seal22 may be attached to the end of the sector 19 adjacent rotatable shaft17. Fluid pressure inlet ports 33 and 35, open into chamber 16, throughcylindrical wall 11 on opposite sides of the sector 19. Ports 33 and 35communicate with fluid output passages 47 and 49 respectively of a valve51 for connection to an external fluid source, both of which are to bedescribed subsequently.

Rigidly secured to rotatable shaft 17 so as to rotate therewith is vane23 extending radially from said shaft to the wall 11 and axiallythroughout the length of chamber 16. A resilient seal 25 may be providedalong two 01 the three outer edges of vane 23, said seal engaging endwall 15 and cylindrical wall 11 as shaft 17 rotates. The third outeredge of vane 23 is secured to a rotatable masking disc 27 described inmore detail below. Vane 23 thus divides chamber 16 into two fluidisolated compartment: which are each respectively defined by a differentside 01 the vane and a different side of sector 19. Each side 0: sector19 also defines the extreme position for the movement of vane 23. Thisis achieved in conjunction witl input fluid received by chambers 16 viainlet ports 33 ant 35 which are positioned adjacent respective oppositeside:

of sector 19. This fluid is trapped between the respective sector sideand the vane 23 as the latter approaches the sector, thereby providing aresilient cushion and avoiding high speed impact between the vane andthe sector.

The radial cross-section of the vane 23 may be any appropriate shapesuch as rectangular, a sector (as best illustrated in FIGURE 2), etc. Aswill be seen from the subsequent description, however, the width of thevane has certain limitations.

Circularly arranged about the center of end wall 13 and extendingaxially therethrough are a plurality of exhaust ports (A1, A2, A3, B1,B2, B3, C1, C2, C3, etc.) which are suitable for conducting pressurizedfluid out of chamber 16. These exhaust ports may take any convenientshape, however, they must be slightly larger than the arcuate length ofthe vane 23 in the region of the ports so that a rather small amount offluid may pass from both sides of the vane through the port over whichthe vane is to be centered. This permits accurate positioning of thevane 23 since a slight deviation of the vane to one side of a centralposition over an open exhaust port will cause a pressure decrease on theother side of the vane, thereby destroying pressure equilibrium acrossthe vane and tending to move the vane towards the central position.Further the exhaust ports must be not appreciably larger than thearcuate length of vane 23 in the region of the ports so that the vanecan be positioned to substantially block passage of fluid through anyindividual exhaust port. In addition, the exhaust ports must be spacedsuch that vane 23 can substantially block only one such port at anytime.

Masking disc 27, secured to vane 23 so as to rotate therewith, iscircular in shape and is coaxial with the shaft 17. The disc 27 has aradius which is just slightly less than the radius of cylindrical wall11, and may employ means such as seal 37 to prevent leakage from chamber16 to exhaust ports A1, B1, C1, etc. Disc 27 passes between sector 19and Wall 13, a relatively tight seal being required to prevent excessiveleakage across the sector 19. To this end there may be provided aradially extending seal 21 secured to the sector 19.

Disc 27 has two cut-outs 39 and 41 which are situated on either side ofvane 23 at a distance from the center of the disc which is substantiallyequal to the radial distance of exhaust ports A1, B1, C1, etc. from thecenter of wall 13. Cut-outs 39 and 41 are slightly larger than oneexhaust port opening but not large enough to encompass two such portopenings when the vane 23 is positioned to block any single port. As aresult of the above structure fluid in chamber 16 normally has access tothe exhaust ports only through cut-outs 39 and 41 in disc 27.

An additional exhaust port 43, provided for reset purposes, extendsthrough wall 11 adjacent to the sector 19. The port 43 is axiallyaligned with the counterclockwisemost port X1 of the aforesaid exhaustports.

A fluid pressure source 53 for the motor feeds fluid at some constantpressure to fluid conducting means 54 and thence to self-centeringshuttle valve 51. The body of the valve 51 is connected via variableorifices 52 and 56 to output passages 47 and 49, respectively, of valve51 which are connected to supply fluid to input ports 33 and 35respectively of the motor 10. Valve 51 comprises a shuttle 65,containing springs 57 and 59 and respective adjustment screws 61 and 63for the springs. The compressive forces on the springs 57 and 59 arenormally adjusted such that for equal back pressures in output passages47 and 49, the shuttle 65 is located in the center of the valve chamber,thereby providing equal blockage of orifices 52 and 56. Thus equalimpedances to flow are provided between the sources 53 and the passages47 and 49. A reduction in back pressure on either end of the shuttlecauses it to move towards that end and block the adjacent orifice 52 or56, thus permitting fluid from the source to be conducted only to theunblocked output passage (47 or 49) which is at the higher pressure.

A series of fluid conducting means A4, A5, A6, B4, B5, B6, C4, C5, C6,etc. are connected respectively to the exhaust ports A1, A2, A3, B1, B2,B3, C1, C2, C3, etc. through appropriate fittings (not illustrated) atthe outer portion of wall 13. As best illustrated in FIGURE 2, the firstof every three sequentially spaced exhaust ports has its respectivefluid conducting means connected to a first manifold, thus fluidconducting means A4, B4, C4, etc. are connected to manifold path 40.Similarly, the second of every three sequentially spaced exhaust portshas its respective fluid conducting means connected to a secondmanifold. Thus fluid conducting means A5, B5, C5, etc. are all connectedto manifold 50. In like manner, the third of every three sequentiallyspaced exhaust ports has its respective fluid conducting means connectedto a third manifold. Thus fluid conducting means A6, B6, C6, etc. areconnected to manifold 60. Each manifold 40, 50, and is provided with adifierent control valve 44, 55, 66 each of which controllably vents itsassociated manifold to reference pressure which is lower than thepressure provided by source 53. If the system operates on pressurizedair, the atmosphere may be used as the lower pressure. Valves 44, 55, 66are individually actuable by controller 67 which may be any appropriatevalve mechanism, either manual or automatic. The valves may be of anyappropriate type, and may be either mechanically or electricallyactuable.

A further fluid conduction means 69 is connected to reset exhaust port43 by appropriate fittings (not illustrated) in end wall 13. Resetcontrol valve 71 controllably vents fluid in conduction means 69 to somepressure lower than that produced by source 53. Reset control valve 71is illustrated as controlled by controller 67 which also providescommands for valves 44, 55, and 66. It is clear, however, that anindependent controller may be employed for reset purposes if desired.

In explaining the operation of the above-described embodiment, it willfirst be assumed that the control valves 44, 55, 66 and 71 are allclosed, thereby precluding existence of any venting path for fluid inchamber 16 on either side of vane 23. Thus the pressurized fluid fromsource 53 has access to the portions of chamber 16 on both sides of thevane, and the equal pressure in both chamber portions is reflected backthrough inlet ports 33, 35 and outputs 47 and 49 of valve 51 to keepshuttle centrally positioned with respect to the valve cylinder. Withthe equal fluid pressures existing on both sides of the vane 23, thevane remains in whatever position in which it was previously placed. Ifit is assumed that this initial position is such that the vane islocated at exhaust port C2 as in FIGURE 2, the cut-outs 39 and 41 ofmasking disk 27 must assume positions whereby they permit fluid inchamber 16 to have access to exhaust ports C1 and C3, respectively.Since the control valves are all assumed to be closed, the fluid inchamber 16 cannot be vented via ports C1 and C3.

Next let it be assumed that control valve 55 receives a command fromcontroller 67 to open. With valve 55 opened exhaust ports A2, B2, C2,etc. are vented; however masking disk 27 prevents all of theseparticular ports except C2 from venting fluid from chamber 16. C2 issubstantially blocked by vane 23 which is centered thereover; thus noappreciable path exists for the fluid in chamber 16 on either side ofvane 23 and the pressure on both sides remains equal. This equalpressure at both output passages 47 and 49 of valve 15 keeps shuttle 65centered and orifices 52 and 56 partially and equally blocked. Vane 23therefore remains stationary in its illustrated position.

Next assume that valve 66 is open, valves 44, 55 and 71 being closed.Under such conditions exhaust ports A3, B3, C3, etc. are vented, but allbut C3 are blocked by masking disc 27. Since C3 is positioned undercut-out 41 in the disc, it is able to vent the pressurized fluid fromthat portion of chamber 16 to the right of vane 23. Since the pressureon the left side of the vane remains unchanged, a pressure diiferentialis created across the vane tending to move shuttle 65 to the right tofurther block orifice 56 and further open orifice 52. In addition, thehigher pressure at the left side of vane 23 moves the vane clockwisetowards exhaust port C3. As the vane begins to block port C3 thepressure to the right of the vane begins to build up, thereby increasingthe back pressure at valve output passage 49. This causes shuttle 65 togradually return to its central position so as to gradually increasefluid flow to the right side of the vane. This proportioning feature ofthe shuttle valve tends to prevent overshoot of vane 23 past the openexhaust port. Once the vane substantially blocks the exhaust port C3,the pressure across the vane is equalized since there is no furtherappreciable exhaust path from chamber 16. Vane 23 thus remainsstationary, and shuttle 65 remains centrally positioned over exhaustport C3.

By similar operation, the motor may be stepped in the counterclockwisedirection. Assume, for example, that the vane is initially positionedover port C2 and that all valves 44, 55, 66, and 71 are closed. If nowvalve 44 is opened so as to provide a vent to ports A1, B1, C1, etc. itis apparent that of these only C1 will not be blocked by disc 27,cut-out 39 being positioned thereover. The pressure on the left side ofvane 23 decreases due to this vent path at port C1 while the pressure onthe right side of the vane remains unchanged. The resulting pressuredifferential moves shuttle 65 to block orifice 52 and moves the vanecounterclockwise until it is positioned over port C1. As discussedabove, shuttle 65 gradually moves back towards its central position asthe vane begins to block port C1, thereby providing a gradual flowincrease to the left side of chamber 16. The gradual flow increaseprevents vane 23 from shooting past port C1 and thus provides astabilizing effect. Once the vane sub stantially blocks C1, masking disk27 blocks all other exhaust ports except B3, C1, C2 and 43 which arerespectively prevented from venting chamber 16 by closed valve 66, vane23, closed valve 55, and closed valve 71. The vane remains stationarydue to the equal pressures thereacross, and shuttle 65 returns to itscentral position.

For any position of vane 23, if valve 71 is opened and valves 44, 55,and 66 are closed, vane 23 will be moved to reset position adjacentabutment member 19 and in blocking relation to exhaust port X1. Thisoccursbecause any time valve 71 is open, chamber 16- is vented via resetport 43, fluid path 69 and valve 71. Since no other valves except 71 areopen, the pressure on the right side of vane 23 exceeds that on the leftside and the vane is forced into counterclockwise movement until it iscentered over vent passage through port 43. It should be apparent thatthe reset position need not be located at an end of the path travelledby vane 23, but may be located anywhere along said path by appropriatelypositioning reset port 43.

It is also possible for vane 23 to be moved rapidly between any twonon-adjacent exhaust ports by sequentially activating valves 44, 55, and66 until the vane moves the required number of steps to the desiredlocation. This may be done automatically by conventional electroniccounting circuitry, or by conventional controls, either or both of whichmay be designed into controller 67. In the electronic type controller,it is evident that conventional computer circuitry may be employed toprogram any desired sequence of positional commands to control valves.

The motor of the present invention is particularly useful in high speedelectric typewriters or printers for stepping a carriage at rates in theorder of 100 steps per second. Such a typewriter is described incopending patent application Ser. No. 501,060 filed on Oct. 22, 1965 inthe name of Ralph P. Means and assigned to the same assignee as thepresent invention.

In a system of this type an electronic counter may I stepped each time asymbol is received and the valve 7 may be activated each time a carriagereturn signal received.

FIGURE 4 illustrates another embodiment of the ir vention is which theexhaust ports are located in cylir drical wall 111 rather than incircular end wall 113 a above. In this embodiment the ports A1, A2, A3,B1, B1 B3, C1, C2, C3, etc. are of any convenient shape, thei size beinglimited by the fact that the outer edge of van 123 must be slightlysmaller than any one port opening These ports are circularly arrangedalong cylindrical waI 111 and are connected according to position inalternat groups of three as above to respective manifolds 1411 150, and160. Pressures in these manifolds are controller by control valves 144,155, and 166, respectively, which it turn are responsive to commandsfrom controller 167 ii a manner identical to that described for theembodimen illustrated in FIGURE 2. Pressure source 153 and valvl 151also operate as described above in providing pres surized fluid tochamber 116 via fluid inlet ports 133 ant 135 located in abutment member119. Similarly, rese control valve 171 and its associated fluid path 169operah to reset the vane 123 by venting fluid from chamber 11( via resetexhaust port 143. The reset exhaust port 143 i: illustrated as beinglocated in abutment member rather than in end wall v113 as above, but itis to be understood that any appropriate location may be utilized.

An annular mask 127 of this embodiment is cylindrical rather thancircular as in the previous embodiment, sc as to conform to the shape ofwall 111 in which the exhaust ports are located. The annulus 127 issecured to the vane 123 at that edge of the vane which communicates withWall 111. It should be noted that for this embodiment, abutment member119 is mounted to the two end walls (not illustrated) of the housing,not to cylindrical wall 111 as in the previous embodiment in order topermit the mask to revolve between abutment member 119' and wall 111.Cut-outs 139 and 141 in the mask 127 permit only the two exhaust portsimmediately adjacent vane 123 to be accessible to fluid in chamber 116.Operation of this embodiment is similar to that of the FIGURE 2embodiment. Opening any of the control valves 144, 155, or 166 creates apressure imbalance across vane 123 until the vane is moved by saidpressure to cover the appropriate venting exhaust port. Annular mask 127prevents more than one of the plurality of exhaust ports associated witheach control valve from venting chamber 116 at any one time.

FIGURE 5 illustrates still another embodiment of this invention whereinthe exhaust ports are arranged in a straight line as opposed to acircle. Housing 210 may be either cylindrical or box shaped. Piston 223-is shaped to conform to the cross-sectional shape of the housing andthus divides chamber 216 into two fluid-isolated compartments. Exhaustports A1, A2, A3, B1, B2, B3, C1, C2, C3, etc. are arranged in astraight line in wall 211 and may be of any size or shape as long asthey are slightly larger than the edge of piston 223. Masking plate 227is contoured to the shape of wall 211 and is secured to the portion ofthe edge of piston 223 which engages the exhaust ports. Mask 227 is ofsuch size and shape that it covers all exhaust ports A1, A2, A3, B1, B2,B3, C1, C2, C3, etc. except the two immediately adjacent the pistonposition, these being coincident with cut-outs 239 and 241. Abutmentmember 219 is located to prevent piston 223 from moving beyond apre-determined reset position when reset control port 243 is actuated byvalve 271. The operation of this embodiment is similar to the operationof the two embodiments described above, with the control valves causingstepped lateral movement of piston 223 and shaft 217 along thelongitudinal axis of the shaft rather than the rotational motion of thevane and shaft created in the previous cases.

It is clear from the above description that this invention provides anovel means for reducing the size, cost and mplexity of fluid-drive steppositioning devices. Spelically, this result is accomplished by reducingthe num- :r of control valves from one per port to a total of three, 1dby grouping the exhaust ports such that one of ery three ports iscontrolled by a common valve. The asking means is employed to ensurethat only the ports ljacent to the wane or piston are eflecti've toproduce raft motion even though one third of all the ports are :tuatedsimultaneously.

It is to be understood that appropriate fluid sealing teams are to beemployed at the various joints throughut the above embodiments so as toprevent undesired uid leakage paths. In addition, various other detailsnd modifications are deemed to be within the scope of 1is invention. Forexample, the invention need not be mited to the particular shuttle valveillustrated in FIG- ]RE 2, but may take the form of any two-output fluid.evice which blocks one or the other output path as a unction ofpressure differential in said paths. Further, he fluid medium to beemployed may be air, oil, or any )ther fluid, and the materials used forthe various elements may be suitable metals, plastics, and the like.

While we have described and illustrated one specific :mbodirnent of ourinvention, it will be clear that variaion of the details of constructionwhich are specifically llustrated and described may be resorted towithout deaarting from the spirit and scope of the invention as deinedin the appended claims.

We claim:

1. A fluid-operated step-positioning device, comprislng:

housing means forming a chamber and having a plurality of exhaust portsdisposed along a determinable path in at least one of its walls;

movable means within said chamber and dividing said chamber into twosubstantially fluid-isolated compartments;

means producing equal fluid pressures in said compartments and acrosssaid movable means for holding the latter stationary;

means producing a fluid pressure diflFerential across said movable meansfor moving the latter along said determinable path in the direction ofleast pressure to block a selected one of said exhaust ports, said meansproducing a fluid pressure differential comprising said plurality ofexhaust ports, three control valve means for selectively venting fluidfrom said chamber via said selected one of said plurality of exhaustports, and fluid path means connecting the first of every threesequentially positioned exhaust ports to a first of said three controlvalves, the second of every three sequentially positioned exhaust portsto a second of said three control valves, and the third of every threesequentially positioned exhaust ports to a third of said three controlvalves;

means for selectively controlling said three control valves;

masking means rigidly secured to said movable means for permittingsubstantial amounts of fluid to be vented from said chamber only via thetwo exhaust ports which :are at any time positioned on opposite sides ofand immediately adjacent to said movable means.

2. The device of claim 1 wherein said equal pressure producing meanscomprises:

a source of fluid pressure;

a fluid inlet means for each of said compartments;

self-centering shuttle valve means having an input and two outputs andfor providing equal pressure at its two outputs when said outputs areequally loaded but zero pressure at one of its outputs when its twooutputs are loaded substantially unequally;

means connecting said source to said valve input and each of said valveoutputs to respective ones of said fluid inlet means.

3. The device of claim 1 further comprising reset means for positioningsaid movable means to some pre-determined initial location along saiddeterminable path.

4. The device of claim 1 wherein said housing is of cylindrical shape,wherein said movable means comprises a vane secured to a shaft which ismovably mounted along the longitudinal axis of the cylinder, and whereinsaid masking means is secured to the edge of the vane which traversessaid determinable path.

5. The device of claim 4 wherein said housing means includes an abutmentmember for determining the extreme positions of said vane.

6. The device of claim 5 wherein said exhaust ports are disposed in acircular path along an end wall of said cylindrical housing and whereinsaid masking means is a circular disk having two cut-outs therein, eachof said cut-outs being large enough to expose only one exhaust port andeach being situated immediately adjacent to and on opposite sides ofsaid vane.

7. A device of claim 6 further comprising reset means for moving saidvane to some pre-deterrnined initial position, said reset meanscomprising:

an additional exhaust port in a cylindrical wall of said housing;

reset control means for selectively venting said chamber via saidadditional exhaust port.

8. The device of claim 6 further comprising reset means for moving saidvane to some pre-determined initial po sition, said reset meanscomprising:

a reset exhaust port located in said abutment member;

reset control means for selectively venting said chamber via said resetexhaust port.

9. The device of claim 5 wherein said exhaust ports are disposed in acircular path around the cylindrical wall of said housing and whereinsaid masking means is cylindrical in shape with two cut-outs therein,each of said cut-outs being large enough to expose only one exhaust portand each being located immediately adjacent to and on opposite sides ofsaid vane.

10. The device of claim 9 further comprising reset means for selectivelyventing said chamber so as to return said vane to a pre-determinedinitial position.

11. The device of claim 1 wherein said movable means comprises a pistonand said housing comprises a cylinder for said piston, wherein saidexhaust ports are disposed on the cylinder wall parallel to thelongitudinal axis of the cylinder, and wherein said masking meanscomprises a plate contoured to the shape of said cylinder wall andhaving two cut-outs therein, each of said cut-outs being shaped toexpose only one exhaust port and each being situated immediatelyadjacent to and on opposite sides of said piston.

12. The device of claim 11 further comprising reset means forselectively venting said chamber so as to return said piston to apre-determined initial position.

13. The device of claim 4 wherein each of said exhaust ports is slightlylarger than said edge of said vane which traverses said determinablepath.

References Cited UNITED STATES PATENTS 1,004,541 9/1911 Martin 91-3572,171,005 8/1939 McNeil et al 91-357 2,398,997 4/1946 Berry et al.91-357 2,954,755 10/1960 Pecchenina 91-357 3,058,450 10/1962 Lissau9l357 PAUL E. MASLOUSKY, Primary Examiner.

